Experimental study on the separation performance of a full-scale SG steam-water separator
Introduction
Steam separator package in nuclear steam generator (SG) is responsible for eliminating water droplets from steam and supplying dry-saturation steam for turbines. Generally, the steam-water separation is done in the upper part of SG and usually consists of three stages, i.e., swirl vane separator for primary separation, steam dryer for secondary separation and gravity separation, which is achieved in the space between the primary and secondary separation (Zhang et al., 2016). The efficiency of the separator package is normally measured in terms of moisture carryover fraction (MCO), which is the mass flowrate of water droplets in the steam and its value should be “less than 0.1%” in an industry-accepted standard (Mauro et al., 1990). Besides the separation efficiency, the other most crucial feature of the steam separator package is its pressure drop, which directly determines the circulation ratio and stability of the circulating two-phase flow (Green and Hetsroni, 1995). With the demands of power density increase for large nuclear power station and compact space for vessel steam generator, the performance of the steam separator package must be significantly improved to assure acceptable steam quality under the conditions of high steam load, operation pressure and circulation ratio (Liu and Bai, 2016, Zhao et al., 2018a).
In the last few decades, a considerable number of experiments have been performed to seek alternatives for separator design and performance improvement. However, as the mock-up tests under prototypical conditions are difficult and costly to implement, previous tests are mostly conducted with the downscaled models in low-pressure air-water systems. Among these, Nakao et al., 2001a, Nakao et al., 2001b measured the liquid-separation rates and pressure drops of a half-scale BWR swirl vane separator while Ikeda et al. (2003) conducted similar experiments using a 45%-scale swirl vane separator. Xiong et al., 2013, Xiong et al., 2014 investigated the effect of structures and flow patterns on the performance of a 45% size-scale AP1000 swirl vane separator, and demonstrated that the tangential slot and annular flow pattern significantly improve the separator performance. Katono et al. (2014) performed a 50% scaled air-water experiment to reduce the pressure drop while maintaining the high separation efficiency of a BWR swirl vane separator. Funahashi et al., 2016, Mike et al., 2012, Kataoka et al., 2008, Kataoka et al., 2009a, Kataoka et al., 2009b, Kataoka et al., 2009c) measured the flow patterns, separation efficiency, pressure drop, liquid film thickness and droplet distribution of a one-fifth scale model swirl vane separator for a BWR to understand the characteristics of two-phase swirling flow and to establish an experimental database for the modeling and validation of numerical methods. In addition, some reduced model tests focusing on the optimization of swirl vanes are also performed to obtain the ideal geometries and features (Jensen et al., 1996, Nakao et al., 2001a, Nakao et al., 2001b, Ikeda et al., 2003, Kataoka et al., 2009c, Matsubayashi et al., 2012, Funahashi et al., 2017). With respect to the steam dryer, the laboratory tests are also carried out to study the performance of wave-type plates, such as the work of Nakao et al., 1998, Nakao et al., 1999, Koopman et al., 2014, Mao et al., 2018 and a series of recent researches by Wang and Tian, 2019a, Wang and Tian, 2019b, Wang and Tian, 2019c, Wang et al., 2020.. On the other hand, as the computation ability increases, many numerical models both in Eulerian-Eulerian framework (Chaki and Murase, 2006, Ogino et al., 2008, Xiong et al., 2014, Liu and Bai, 2017) and Eulerian-Lagrangian framework (Zhang and Bo, 2015, Zhang et al., 2016, Zhang et al., 2017, Zhao et al., 2018a, Zhao et al., 2018b, He et al., 2019, Fang et al., 2020) have been developed to simulate the flow characteristics and separation performance of swirl vane separator and dryer. In these numerical models, the water droplets are assumed to be uniformly distributed and the behaviors of collision, coalescence, break-up and re-entrainment of droplets are usually neglected. Meanwhile, the droplets are considered to be advected by the steam with no speed difference, which however is significantly different from the actual condition in real swirl vane separator and dryer.
From the review above, although the separation performance of swirl vane separator and dryer has been widely studied but with little experimental verification under prototypical conditions. As the separation characteristics vary for different working fluids and the separator performance changes under different operation conditions, a complete knowledge of the performance of separator package in nuclear SG and the relationship between each separator stage have not been achieved, and further investigation is still required in this area. In the present paper, a unique steam-water two-phase flow system is designed and built to study the through separation performance of a full-scale SG separator package of the GEN-III China advance PWR. The saturated steam and water at a rated pressure of 6 MPa and temperature of 275.6 °C s are used as working fluids, and the Reynolds number of steam and water is in the range of 1 × 106–6 × 106 and 8.27 × 105–3.2 × 106, corresponding to power loads of 25%–145% of nominal load under prototypical conditions. The details of separation efficiency, moisture carryover and pressure drop of swirl vane separator, gravity separation space and steam dryer are obtained systematically. Also, the contribution of each stage of the separator package in nuclear SG and the relationship between them are gained for the first time.
Section snippets
Experimental setup
A scheme diagram of the high-pressure steam-water two-phase flow system is shown in Fig. 1. It is uniquely designed so that the full-sized steam separator package up to 500 mm ID can be tested at full scale and under prototypical conditions. This system is adiabatic and mainly composed of two circuit loops, i.e., the steam loop and the water loop, which have a mixer and a test section. The steam loop consists of a steam compressor, two control valves, one flowmeter and an external separator.
Separation efficiency η
Fig. 5 shows the separation efficiency of each stage of the separator package. In the figure, the power level based on the steam flowrate are also shown. For swirl vane separator shown in Fig. 5(a)-(b), the separation efficiency of swirl vanes ηswirl increases obviously with Reg first and then inclines to stable value that ranges between 85% and 94% at different Rel. This implies that for a given water flow, when Reg reaches a critical value, the further increase of Reg makes little
Conclusions
In this paper, steam-water two-phase experiments are carried out to study the through separation performance of a full-sized steam separator package of the GEN-III China advanced PWR, at a rated pressure of 6 MPa and temperature of 275.6 °C. Corresponding to 25%–145% power loads under prototypical conditions, the separation efficiency, moisture carryover and pressure drop of the swirl vane separator, gravity space and steam dryer are obtained and compared, when steam and water Reynolds number
CRediT authorship contribution statement
Li Liu: Funding acquisition, Investigation, Formal analysis, Writing - original draft. Bingbin Ying: Conceptualization, Methodology, Formal analysis. Hanyang Gu: Conceptualization, Supervision, Writing - review & editing. Dehui Xu: Investigation. Chao Huang: Investigation. Shuo Chen: Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51906147). L. Liu was also supported by the China Postdoctoral Science Foundation (Grant Nos. 2019 T120341 and 2019 M651495).
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